Can we talk Carbs?
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Tuned
Joined: Jul 2010
Posts: 432
Likes: 34
From: West Fort Worth, TX
Can we talk Carbs?
Fuel not Food. Trying so size my build right. Currently have a 600 CFM. One suggestion here was 650. Knowledgable Summit Racing guy suggested my current 600 was fine. Holley website suggested 650. My looking online (hotrod.com and others) learned me some formulas that provide various CFMs. cubic inch x max rpm / 3456 (357 (351 bored 30 over) x 6000 (acccording to cam specs (one website suggested adding 200 rpm for overspeed) / 3456 = 620 CFM). But then you need to times this by the effeciency which is 85% equaling 526 CFM. 100% is race car, 85% and less is stock and in between (me) is 85%-90%. I thought 526 CFM was the answer just sticking with my 600 CFM but I learned a new formula accounting to vacuum loss from the carbs max CFM before it’s usable. I read 1.5 inch - Hg (whatever that means) is a typical loss through accessories (brake booster and distro). Race engines with little vacuum loss would expect .3 loss and my application should be .8 to 1.0 (??). So the addition to the above formula is dividing the CFM by the square root of .9/1.5. This would equal 680 CFM. If I rounded this number or just added the 200 rpm over speed it is around 700 CFM. Just asking what your thoughts are on the subject. Thanks, my relavent specs are below.
351W
Com cam 230 dur @ .050 and .544 lift (2,500-6,000)
afr 165s
airgap
plan for headers with 1 5/8 pri and 2 1/2 exhaust
roller retrofited
351W
Com cam 230 dur @ .050 and .544 lift (2,500-6,000)
afr 165s
airgap
plan for headers with 1 5/8 pri and 2 1/2 exhaust
roller retrofited
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
Size the carb for how it will be driven mostly, if the only thing you care about is max HP then bigger is better but if you want better throttle response and street manners then the one you have is good IMO.
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
600 is fine for it. What you don't want is a carb that's too big in terms of CFM rating. You've got to remember that it's the pressure differential that pulls the fuel through the circuits to feed the beast. And with a carb that's too big at WOT you loose that. The CFM rating also is just that: a rating at 1.5"HG. What your engine combo will generate at WOT may be more or less than that, so the flow rating will be more or less than the listed rating. The center carb on my 331 is rated at 250 CFM, but doing the math, it flows more like 350 cfm just before the two secondary carbs start to open at around 4500 rpms (Progressive linkage and gradually giving it more pedal)
Elder User
Joined: Nov 2003
Posts: 778
Likes: 2
From: southeast Idaho
You have a known good carb, run it for a while and see what you think. The 600 holley if thats what it is has very good street manners and excellent low end response. If you run it up too 6,000 rpm yeah its going to start to peter out before you quite get there so if that is more important get the 650 or 750.
But I would still run the known carb first
But I would still run the known carb first
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Fleet Owner
Joined: Sep 2005
Posts: 28,609
Likes: 18
From: Lake Tahoe, Nevada
Pretty good advice here. Run what you have and then see what needs fixing. I also like vacuum secondaries, especially like a Quadrajet, tiny primaries that give excellent throttle response and giant secondaries that open as much as there is demand for.
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